(5.) _Impenetrability._--This property will be most clearly explained by defining the positive quality from which it takes its name, and of which it merely signifies the absence. A substance would be _penetrable_ if it were such as to allow another to pass through the space which it occupies, without disturbing its component parts. Thus, if a comet striking the earth could enter it at one side, and, passing through it, emerge from the other without separating or deranging any bodies on or within the earth, then the earth would be penetrable by the comet. When bodies are said to be impenetrable, it is therefore meant that one cannot pass through another without displacing some or all of the component parts of that other. There are many instances of apparent penetration; but in all these, the parts of the body which seem to be penetrated are displaced. Thus, if the point of a needle be plunged in a vessel of water, all the water which previously filled the space into which the needle enters will be displaced, and the level of the water will rise in the vessel to the same height as it would by pouring in so much more water as would fill the space occupied by the needle. (6.) _Figure._--If the hand be placed upon a solid body, we become sensible of its impenetrability, by the obstruction which it opposes to the entrance of the hand within its dimensions. We are also sensible that this obstruction commences at certain places; that it has certain determinate limits; that these limitations are placed in certain directions relatively to each other. The mutual relation which is found to subsist between these boundaries of a body, gives us the notion of its _figure_. The _figure_ and _volume_ of a body should be carefully distinguished. Each is entirely independent of the other. Bodies having very different _volumes_ may have the same _figure_; and in like manner bodies differing in _figure_ may have the same _volume_. The figure of a body is what in popular language is called its _shape_ or _form_. The volume of a body is that which is commonly called its _size_. It will hence be easily understood, that one body (a globe, for example) may have ten times the volume of another (globe), and yet have the same figure; and that two bodies (as a die and a globe) may have _figures_ altogether different, and yet have equal _volumes_. What we have here observed of volumes will also be applicable to lengths and areas. The arc of a circle and a straight line may have the same length, although they have different figures; and, on the other hand, two arcs of different circles may have the same figure, but very unequal lengths. The surface of a ball is curved, that of the table plane; and yet the _area_ of the surface of the ball may be equal to that of the table.